2024: d07: ex2: add solution

2024/d07/ex2/ex2.py

@@ -0,0 +1,41 @@
+#!/usr/bin/env python
+
+import operator
+import sys
+
+
+def solve(input: str) -> int:
+    def parse(input: list[str]) -> list[tuple[int, list[int]]]:
+        return [
+            (int(value), [int(n) for n in numbers.split()])
+            for value, numbers in map(lambda l: l.split(": "), input)
+        ]
+
+    def solvable(target: int, numbers: list[int]) -> bool:
+        def concat(lhs: int, rhs: int) -> int:
+            return int(str(lhs) + str(rhs))
+
+        def helper(current_value: int, current_index: int) -> bool:
+            next_index = current_index + 1
+            if len(numbers) == next_index:
+                return target == current_value
+            if current_value > target:
+                return False
+            for op in (operator.add, operator.mul, concat):
+                if helper(op(current_value, numbers[next_index]), next_index):
+                    return True
+            return False
+
+        return helper(numbers[0], 0)
+
+    equations = parse(input.splitlines())
+    return sum(value for value, numbers in equations if solvable(value, numbers))
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()